The reduction of cooling time, i.e., the time necessary for cooling a heating furnace when it is dormant due to either an accident or a regular inspection, is an important point in increasing efficiency of an installation for rolling steel materials.
For a conventional heating furnace for use in hot rolling, JP-A-Sho50-86408 and JP-A-Hei4-280913 (the term "JP-A-" as referred herein signifies an "unexamined published Japanese Patent application") disclose that it is necessary to lower the temperature to ambient temperature before allowing an operator to enter into the heating furnace, and that the cooling time usually takes from about 24 hours to a period of from about 4 to 6 days. However, the details of the method for cooling the heating furnace are not disclosed. Probably, air cooling using a cooling fan described below is employed. The cooling method conventionally employed by the present inventors comprises air cooling alone at a cooling air flow rate of from 70,000 to 80,000 Nm.sup.3 /Hr, which corresponds to the total of the air produced by a cooling fan taken from the extraction port of the heating furnace and the cooling air from burners. If the scale deposited inside the furnace has a large heat accumulation (and particularly, in case the soaking zone has a large heat accumulation), the scale deposited inside the furnace could not be readily cooled, and it requires 2 or 3 days, or a period even longer, to cool the furnace itself.
In JP-A-Sho63-28817 is disclosed a technology which comprises rapidly cooling the furnace body of a converter immediately after discharging steel, thereby allowing quick thickness measurement of the refractories which make the inner wall of the furnace body. More specifically, it comprises quenching the bricks by directly blowing a water mist to the refractories inside the converter. Furthermore, fire resistant materials are added into the mist to prevent spalling from occurring on the bricks constituting the inner wall of the furnace. However, the method described in this reference comprises spray cooling a particular portion that is thereafter subjected to the thickness measurement of the refractories. Thus, this is a non-uniform cooling by means of a partial forced cooling, and damage is suspected to occur due to thermal stress. Moreover, the method of this reference is not a technology related to the cooling of a heating furnace for use in hot rolling. That is, it does not relate to a method for cooling the scale deposited inside the furnace, nor to a method or apparatus for rapidly cooling the entire heating furnace over a large area.
JP-A-Sho62-136518 refers to a method and an apparatus for removing the scale deposited on a skid of a walking beam type heating furnace by using a highly compressed air. It comprises a plurality of air nozzles arranged in correspondence with the skids along the width direction of the furnace, which are moved in the longitudinal direction of the furnace by using a walking beam. However, only air piping are provided in this case because this technique does not relate to a technique for cooling the furnace.
Thus, to overcome the aforementioned problems, there is proposed a method which comprises increasing the number of cooling fans that are set on the extracting port side, thereby increasing the total rate of cooling air (i.e., to 200,000 Nm.sup.3 /Hr or higher) inclusive of the air provided from the burner, or a method which comprises spraying a coolant against the scale deposits that are formed inside the furnace.
Still, however, the method which comprises increasing total flow rate of cooling air by increasing the number of cooling fans provided on the extracting port side suffers a problem as such that seasonal factors greatly influence the cooling power. More specifically, a limit has been found in shortening the cooling time in summer seasons, because the temperature of the cooling air remain high in summer. On the other hand, the method of spraying a coolant against the scale deposits that are formed inside the furnace has found that damages occur on the refractories because water is scattered over the refractories when the refractories are still at high temperatures. To circumvent such damages from occurring, water must be sprayed only after the inner furnace temperature is lowered to a sufficiently low level, and this requires a long waiting time which, as a result, makes it impossible to shorten the cooling time to a satisfactory level.
The present invention has been achieved with an aim to overcome the aforementioned disadvantages, and an object of the present invention is to provide a method and an apparatus for cooling a heating furnace, which surely shortens the cooling time of the furnace irrespective of seasonal conditions and yet, without damaging the refractories inside the furnace.